FA synthesis and alternative oxidation-Brar 12 5 Flashcards
What is the reducing power in the cell and how is it produced?
NADPH
produced by either the pentose phosphate pathway or malic enzyme
Malic enzyme:
converts malate into pyruvate producing a CO2 and and NADPH
(citrate from the TCA cycle–> malate–> pyruvate)
NADPH=important in FA synthesis
What are the enzymes required for FA synthesis? What are their structures and any cofactors required for their function? (1st only)
FA synthase
FA elongase
FA desaturase
acetyl CoA –> malonyl CoA (by acetyl CoA carboxylase. REQUIRES biotin and ATP to bind the CO2 to biotin). also, the rate limiting step of FA synthesis
FA synthase Adds 2 carbon units from malonyl CoA to the growing chain to form palmitate (16:0)
FA synthase is a large enzyme made up of a homodimer
Each subunit has 7 catalytic activities and an acyl carrier protein segment
ACP segment contains a phosphopantetheine residue
A 4 carbon keto group is produced which is reduced in a series of 3 reactions
The 4 carbon fatty acyl chain is then transferred to a cysteine sulfhydryl group and subsequently condenses with malonyl CoA
This is repeated until the chain is 16 carbons long and that point the chain is hydrolyzed and palmitate is released
• In liver palmitate and other FA’s are converted into triglycerides and packaged up into VLDL for transport
Regulation of fat synthesis
malonyl CoA inhibits carnitine palmitoyltransferase I
Malonyl CoA levels are elevated when acetyl CoA carboxylase is activated thus mitochondrial beta oxidation is inhibited
This prevents a futile round of FA synthesis going on while beta oxidation is also on
Acetyl CoA carboxylase is a monomer in its inactive form and comes together to form multimeric complexes in its active form
• Regulated by covalent modification and allosteric means.
o Allosteric activation by citrate.
o Allosteric inactivation by long-chain fatty acyl CoA.
o Inactivated by phosphorylation in presence of glucagon.
o Activated by dephosphorylation in presence of insulin.
FA Elongation
Palmitate can be activated into palmityl CoA which can be elongated
This occurs in endoplasmic reticulum
Malonyl CoA is the 2 carbon donor
Requires NADPH
Similar to fatty acid synthesis except the fatty acyl chain is attached to ***coenzyme A ( instead of the phosphopantetheinyl residue of ACP)
The major product is stearic acid (18:0)
Longer chains can also be produced
FA Desaturation
via Fatty acyl CoA desaturase
Requires molecular oxygen, NADH and cytochrome b5 ***
Occurs in endoplasmic reticulum
Most common desaturation involves the formation of a double bond between carbon 9 and 10 which produces palmitoleic acid (16:1, 9)
Another common conversion is stearic acid into oleic acid (18:1, 9)
Other positions which can be desaturated in humans include carbon 4, 5 and 6
Eicosanoids
o Polyunsaturated FA with double bonds 3 carbons from methyl end and 6 carbons from methyl end are required for the synthesis of eicosanoids
o Cannot be synthesized de novo by humans (i.e. from glucose via palmitate)
o Dietary sources from dietary plant oils which contain linoleic acid (18:2, Δ9, 12) and α−linolenic acid (18:3, Δ9, 12, 15)
o In humans linoleic acid can be elongated and desaturated to arachidonic acid (20:4, Δ5, 8, 11, 14)
• Uses NADH.
o α−linolenic acid similarly can be converted into eicosapentaenoic acid (20:5, Δ5, 8, 11, 14,17)
what are some common Omega-3 Fatty Acids and their dietary sources?
o Optimal dietary source is fatty fish.
• Salmon, albacore tuna, mackerel, herring, lake trout, sardines
o Non-animal sources include canola, soybean, walnut and flaxseed.
o Fish don’t actually make the omega-3 fatty acids themselves.
• The compounds come from the algae they consume.
o α-linolenic acid
o Stearidonic acid (SDA)
o Eicosapentaenoic acid (EPA)
o Docosahexaenoic acid (DHA)
o Algae genes have been introduced into canola to produce DHA
• Unstable with strong odor.
o Soybean has also been altered to produce SDA
• Precursor for longer chain FA’s
Peroxisomal oxidation of long chain straight FA
o Exclusive site of very long chain FA oxidation (24-26 carbon) and:
• The CoA esters of eicosanoids
• 2-methyl-branched fatty acyl-CoAs
• CoA esters of the bile acid intermediates di- and trihydroxycoprostanoic acids.
o Oxidation stops when a 4-6 carbon fragment is generated.–> go to mitochondria for beta oxidation
o Some long chain FA can also be used in this manner (20 carbons).
o The long chain FA acyl CoA synthetase in the peroxisomal membrane does NOT require carnitine to translocate acyl CoA derivatives across the membrane.
• This is how it differs from β-oxidation in the mitochondria.
2 carnitine acyltransferases were found in rat liver peroxisomes
o The first enzyme is acyl CoA oxidase which directly transfers electron to oxygen producing hydrogen peroxidase
• No energy produced in first step.
• Requires FAD. electrons from FAD–> FADH2–>H20–> H2O2 *peroxisomal oxidation
o Different genes encode for the enzymes which proceed with steps similar to beta oxidation after initial step
o NADH and acetyl-CoA are produced.
Peroxisomal β -oxidation cycle of straight-chain fatty acyl-CoA enzymes: 1 palmitoyl-CoA oxidase 2 (2E)-enoyl-CoA hydratase-1 3 (2E)-enoyl-CoA hydratase-2 4 (3S)-hydroxyacyl-CoA dehydrogenase 5 (3R)-hydroxyacyl-CoA dehydrogenase 6 3-ketoacyl-CoA thiolase
Peroxisomal oxidation of long chain branched FA
Phytanic acid and pristanic acid are the most common dietary LCBFA’s
Breakdown products of chlorophyll and consumed through green vegetables
Not found in animals
Oxidized in peroxisomes to a 8 CARBON BRANCHED FA which is then degraded further in the mitochondria
oxidation of phytanic acid is Normally a minor pathway–>Upregulated in MCAD deficiency
omega- Oxidation
Normally a minor process in metabolism
FA are oxidized at omega end by endoplasmic reticulum enzymes
Process utilizes cytochrome P450, molecular oxygen and NADPH to oxidize the omega carbon to an alcohol
A dehydrogenase then converts the alcohol into a carboxylic acid
Omega oxidation produces DICARBOXYLIC ACIDS which can be utilized in beta oxidation
Regulation of peroxisomal alpha and beta and microsomal omega oxidation
Peroxisomal alpha and beta oxidation and microsomal omega-oxidation are not feedback regulated
They function mainly to lower levels of water insoluble compounds which resemble FA’s which are toxic to cells
Rate is regulated by availability of substrate
